Production risers, also known as marine risers and subsea risers, are used in offshore hydrocarbon production facilities to convey produced fluids including hydrocarbons from a subsea well to a topsides production platform or vessel. Production risers are designed and sized to meet specific mechanical requirements, e.g., fatigue life, taking into account anticipated field conditions including flow regime, fluid velocity, pressure, temperature and fluid chemistry. Excessive riser fatigue can lead to reduction in the life-span of the riser and riser replacement.
Design requirements for production risers change over the life of an oil and gas field, as produced fluid composition and flow regimes change. For one thing, over the life of the field, fluid velocities in a production riser decrease as flow of production fluids from a well diminishes over time. Pressure and temperature profiles of the produced fluids can change over time. Additionally, the need for corrosion resistance of the production riser may increase as a reservoir sours over time. The brownfield tie-in of other reservoirs having increased level of sour gas, i.e. carbon dioxide and hydrogen sulfide, can also increase the corrosivity of the production fluids seen by the production riser. As a result of corrosive or erosive fluids flowing through a riser, the riser may experience a reduction in wall thickness to the point that its mechanical properties are affected and it must be replaced. The need for gas lift in the production riser may increase. In certain instances, the optimum riser design parameters, e.g., internal diameter and selected material to meet the requirements for late life operations can be significantly different from the parameters required for early life operations. In such instances, a production riser may be required to be replaced during the life of the field.
The most common solution is to replace the production riser with a riser having a different diameter and/or different metallurgy. Such riser replacement programs require offshore installation or pipelay vessels, resulting in high capital and/or operating expense. The riser design, procurement and installation process is generally time-consuming, taking up to 24 months, depending on factors such as the degree of remoteness of the production facility, water depth, pressure, fluid composition and flowrate. Some facilities include additional riser slots to allow for the installation of a different riser once the initial production riser becomes unsuitable for use, resulting in additional capital expense.
It would be desirable to avoid having to replace a production riser during the life of a field.